1. Field of the Invention
The present invention is directed to a perpendicular magnetic recording medium. In particular, the present invention is directed to a perpendicular magnetic recording medium that includes a structurally discontinuous magnetic capping layer over a perpendicular magnetic recording layer for improved magnetic recording properties, including an increased signal-to-noise ratio (SNR), improved thermal stability and reduced transition jitter.
2. Description of Related Art
Thin film magnetic recording media are composed of multiple layers, including one or more magnetic recording layers, disposed on a substrate. Traditionally, the magnetic recording layer includes small magnetic grains that have an easy magnetization axis that is magnetically oriented longitudinally (i.e., in plane) with respect to the magnetic layer.
The areal density of longitudinal magnetic recording media has been increasing at a compounded growth rate of about 60% per year and areal densities as high as 100 gigabits per square inch (Gbit/in2) have been demonstrated. Scaling longitudinal recording media to higher areal densities requires smaller magnetic grains. However, as the grain size is reduced, thermal fluctuations can cause the magnetic domains to “flip”, causing a loss of magnetization over a period of time. Media having a higher magnetic coercivity and an increased track density (tracks per inch, or TPI) can mitigate this problem. However, the large write head gaps that are needed for good overwrite of high coercivity media lead to excessive fringing, negatively affecting the data written on adjacent tracks.
Perpendicular (vertical) magnetic recording media have been proposed as a way to increase areal densities beyond 100 Gb/in2 Perpendicular magnetic recording media include a magnetic recording layer having an easy magnetization axis that is substantially perpendicular to the magnetic layer. A perpendicular write head, such as a monopole write head or a shielded pole write head, is utilized to magnetize the grains in the perpendicular recording layer. Examples of perpendicular recording media and perpendicular write heads are disclosed in U.S. Pat. No. 4,656,546 by Mallary and U.S. Pat. No. 4,748,525 by Perlov, which are incorporated herein by reference in their entirety.
Oxygen-doped magnetic alloys are promising materials for use as the hard magnetic recording layer in perpendicular magnetic recording media. CoPtCrO is an example of an oxygen-doped magnetic alloy that can be fabricated with small grains having good magnetic decoupling and good thermal stability. Magnetic layers made from CoPtCrO include magnetic grains of Co separated by a non-magnetic oxide phase that preferentially segregates to the grain boundaries.
It is desirable to form a magnetic recording layer with a high magnetic anisotropy (Ku), which is a measure of the magnetic stability of the grains. However, the fabrication of well-isolated Co grains with high anisotropy is difficult because the magnetic grain size, as well as the degree of grain boundary segregation, is highly dependent upon the level of oxidation. At high oxidation levels, the oxygen can incorporate into the Co grain, resulting in reduced anisotropy. Moreover, variations in the degree of grain boundary segregation within the magnetic layer result in a distribution of exchange fields and magnetic cluster size, which increases transition jitter.
Transition jitter is a major component of medium noise, which is a limiting factor in increasing the recording density of magnetic recording media. In magnetic media, transition jitter arises at the junctions between zones of opposite magnetic orientation. At these junctions, the magnetic orientation transitions are recorded at the edge of the magnetic units that make up the medium, i.e., at the edge of the grains. However, the magnetic grain edges are not aligned along a straight line. As a result, the magnetic transitions do not occur abruptly and local deviations of magnetization from the intended transition center are present.
Coupled granular/continuous (CGC) media have been proposed to improve transition jitter through transition smoothing. Specifically, exchange-coupled continuous (i.e., amorphous) films such as Co—Pt, TbCoPr, TbCo and Pt-rich CoPtCr have been used as continuous layers over an underlying granular magnetic film to magnetically pin the magnetic grains.
Muraoka et al., “Analysis on Magnetization Transition of CGC Perpendicular Media,” IEEE Transactions On Magnetics, Vol. 38, No. 4, July 2002, discloses the use of CGC media to improve transition jitter through transition smoothing. The CGC media disclosed by Muraoka et al. include a first layer of granular CoCrPt having a magnetically isolated grain structure. A second layer of structurally continuous Co—Pt with no pinning sites and having a thickness of 3 to 6 nanometers is deposited on top of the first granular layer. The two layers are magnetically coupled by interlayer exchange coupling. Muraoka et al. disclose that the CGC media show reduced transition jitter due to the interlayer exchange coupling between the granular magnetic layer and the continuous layer.
U.S. Pat. No. 6,716,516 (Futamoto et al.) discloses a perpendicular magnetic recording medium that includes tow perpendicular magnetization films having different magnetic anisotropy constants. A first perpendicular magnetization film is a granular layer of a Co-based alloy containing at least one element selected from Cr, Ta, Pt, Pd, Si, V, Nb, W, Mo, Hf, Re, Zr, B, P and Ru. A second perpendicular magnetization film is deposited on the first film and is either: i) a multi-layered perpendicular magnetization film of CoPt, CoPd or alloys thereof; or ii) an amorphous perpendicular magnetization film, such as TbFeCo containing rare-earth elements. The second film has a greater magnetic anisotropy in the perpendicular direction than the first film. Futamoto et al. disclose that the medium has reduced noise due to strong exchange coupling in the longitudinal direction of the second film, which reduces reverse magnetic domains and microscopic instability that is typically present at the surface of the first film.
U.S. Pat. No. 6,794,028 (Uwazumi et al.) discloses perpendicular magnetic recording media that include two magnetic layers. A first lower magnetic layer is made from CoCr magnetic grains and non-magnetic grain boundaries of oxides or nitrides. A second upper magnetic layer having high anisotropy includes an amorphous alloy of a transition metal selected from Ni, Fe and Co and a rare earth element selected from Pr, Nd, Gd, Tb, Dy, and Ho in a concentration of from 10 at. % to 35 at. %. A perpendicular magnetic recording medium having the disclosed structure exhibits favorable electromagnetic conversion characteristics under a high recording density condition and excellent thermal stability.
U.S. Pat. No. 6,183,893 by Futamoto et al. discloses a perpendicular magnetic recording medium that includes a magnetized film with a bilayer structure. In one embodiment, Futamoto et al. disclose that an upper layer of the magnetized film, which can be hetero-epitaxially grown on the lower layer, has a lower concentration of nonmagnetic elements than a lower layer of the magnetized film. Additionally, the saturation magnetization (Ms) and magnetic anisotropy (Ku) of the upper layer are larger than the lower layer. Futamoto et al. disclose that the media exhibits reduced noise.
There remains a need for improved perpendicular recording media having improved signal-to-noise ratio (SNR), transition jitter, resolution and thermal stability.